System for processing an audio signal
This invention relates to a system for processing an audio signal to reduce the noise contained therein, the system comprising a source of an audio signal in electric form, a first filter group for dividing the audio signal into several subband signals in different frequency bands, a detector for each subband signal to detect the audio signal level thereof, and a compressor/expander for compressing/expanding each subband signal on the basis of the detected audio signal level thereof, and a second filter group for combining the compressed/expanded subband signals to generate a reconstructed audio signal, whereby the first filter group is arranged to divide the sound frequency range into bands with a width enabling the processing of the signal within the critical bands.
The above system for reducing the noise of an audio signal excluding the use of the critical bands corresponds to what is known as Dolby SR noise attenuation system, in which five independently controllable noise attenuation channels are used to improve the signal to noise ratio separately in five different frequency bands. Thus the reproduction range of the sound reproduction system is divided into five ranges, each of which is processed separately. The basic principle of the Dolby noise attenuation system is to improve the signal to noise ratio such that the lower the level of the audio signal, the more the audio signal is amplified during the recording. This procedure in which the dynamic range of the audio signal is narrowed is called compression. An audio signal compressed in this manner must be expanded upon reproduction, whereby the dynamic range thereof is restored almost to its original range. The advantage of the method is the attenuation of the noise in the transmission link or recording means by about 10 dB. The primary disadvantage of the Dolby system is that the noise can be attenuated by the system only upon reproduction of audio signals that are compressed in accordance with the system concerned. In addition, the division of the frequency bands, if such a division is used, is very rough.
U.S. Pat. Nos. 4,896,362 and 5,105,463 teach a system in which a digital audio signal is split into subbands of approximately critical bandwith in order to make use of the noise-masking curve of the human auditory system by removing the masked portion of the signal. The final aim of the system is to requantize the signal within each subband to a minimum number of bits so that the quatization noise would not be audible. U.S. Pat. No. 4,882,762 discloses a system in which an audio signal is frequency selectively compressed so that the compression ratio is selected independently for each of the frequency bands. These systems are, however, not intended for attenuating background noise present in an audio signal.